1. Field of the Invention
This invention relates to an optical transceiver with an optical subassembly and, in particular, to an optical transceiver that allows an improvement in assembly workability, a reduction in assembly failure and an enhancement in heat radiation performance.
2. Description of the Related Art
An optical transceiver for optical communications includes an optical transmission module and an optical reception module which are placed in a housing. The optical transmission module and the optical reception module are similar in mechanical structure except that its electrical function is reversed between transmission and reception. In detail, the optical transmission/reception module is composed of a package with a light-emitting element or a light-receiving element (hereinafter called optical element) placed therein and an optical system such as a lens tube. The package may include a peltiert element for cooling the optical element. The optical element and the peltiert element are formed as a semiconductor chip and, therefore, hereinafter collectively called chip. The housing is provided with a window through which one end of an optical fiber as a transmission line is inserted. In the window, a receptacle is disposed to accurately place the optical fiber in the optical path. Although the receptacle is a simple tube, it is an important component to define the optical path toward the optical element in its inside space. By previously uniting the receptacle with the optical transmission/reception module, the assembly process of an optical transceiver can be simplified. Hereinafter, such united components are called optical subassembly.
As shown in FIG. 1, a conventional optical subassembly 41 is constructed by uniting a cylindrical built-in chip portion 41a with a chip for light transmission/reception built therein with a receptacle portion 41b protruding from one end of the built-in chip portion 41a to define the optical path. In order to position and secure the optical subassembly 41 into a housing 43 (partly shown in FIG. 1), a subassembly holding member 44 is used. The subassembly holding member 44 is integrally provided with a front restriction portion 44b with a hole into which the receptacle portion 41b and a base (not shown) on which the built-in chip portion 41a is mounted. The receptacle portion 41b has a flange 41c formed midway with a large diameter. The hole of the subassembly holding member 44 is provided with a groove into which the flange 41c is fitted. Thus, the optical subassembly 41 is positioned and secured by the subassembly holding member 44. Meanwhile, the form of the base is detailed in Japanese patent application laid-open No. 2004-103743. The base is provided with a roundly-recessed receiving portion so as to surely receive the cylindrical built-in chip portion 41a. Alternatively, the other subassembly holding member 44 may be composed of only the front restriction portion 44b without having the base. This type is as shown in FIGS. 2 and 3.
In the structure as shown in FIG. 1, the bottom face of the subassembly holding member 44 is located lower than the bottom face of the optical subassembly 41. This structure is based on the concept that the optical subassembly 41 is held by the subassembly holding member 44 and the subassembly holding member 44 is secured to the housing 43. The optical subassembly 41 is not directly in contact with the housing 43.
On the other hand, the optical subassembly 41 is provided with leads (not shown) electrically connected to electrodes of the chip and exposed at the other end on the side of the built-in chip portion 41a. The leads are soldered to a board (not shown) to interconnect each other.
The process of assembling the optical receiver is conducted such that, as shown in FIG. 2, a board 2 is previously connected to an optical subassembly 51, a subassembly holding member 54 is attached into the optical subassembly 51, and these are installed in a housing 53. The subassembly holding member 54 is vertically divided into two parts which can hold the optical subassembly 51 while sandwiching the receptacle portion.
As seen from the above assembly process, the optical subassembly 51 and the board 2 are in integration form installed into the housing 53. However, the optical subassembly 51 and the board 2 are connected only by soldering leads for electrical connection to electrodes on the board 2, and, thus, the optical subassembly 51 and the board 2 are not integrated rigidly. In addition, the optical subassembly 51 elongates in a direction away from the lead because of having the receptacle portion, and the board 2 also elongates in the opposite direction. Therefore, when the optical subassembly 51 and the board 2 are in integration form transported to be installed into the housing 53, both of them are kept unstable. It is apparent that strain force caused by a vibration or weight balance during all that time will concentrate on the soldered portion.
The strain force may cause the breaking of leads or the assembly failure and, therefore, the product yield of the optical transceiver will lower. Further, even if such a visible failure is not generated, when the lead or solder has a crack or the chip in the built-in chip portion is subjected to an unnecessary strain force, the reliability will lower due to a failure in the communication and durability performance.
On the other hand, since in high-speed and large-capacity communications, the chip has an increase in consumed power and thereby has an increase in heat generation, a heat radiation member is needed that can efficiently discharge heat from the optical subassembly to the housing. Thus, as shown in FIG. 3, the subassembly holding member 44 is not directly in contact with a bottom 63e in the housing and a heat radiation sheet 8 is disposed between the subassembly holding member 44 and the bottom 63e. 
The optical subassembly 41 is not in close contact with the heat radiation sheet 8 since it has a gap in bottom level relative to the subassembly holding member 44. However, since the heat radiation sheet 8 has a finite thickness and some elasticity, when the subassembly holding member 44 and the optical subassembly 41 are pressed down by a lid (not shown) of the housing or by an upper base opposite to the lower base, the optical subassembly 41 becomes in close contact with the heat radiation sheet 8 while the heat radiation sheet 8 under the subassembly holding member 44 is crushed. Thus, depending on the balance of press-down force, the entire housing or the subassembly holding member 44 or optical subassembly 41 is subjected to a strain force. The strain force is applied to the connection part of the lead 46 and the board 2 and thus concentrates on the soldered portion.
Further, the force is also applied to the optical element or optical system in the optical subassembly 41. Therefore, the optical output may decrease due to a deviation in the optical axis. As a result, the reliability lowers.